Method and apparatus for video coding

ABSTRACT

A method for video decoding at a decoder is provided. In the method, reconstructed samples of a reconstructed block of a picture are stored in a first reference sample memory. The first reference sample memory is configured to store at least one set of a number of luma samples and corresponding chroma samples of the reconstructed block. Further, reconstructed samples of a current block of the picture are stored in a second reference sample memory. The second reference sample memory is configured to store only one set of the number of luma samples and corresponding chroma samples of the current block. A current sub-block in the current block is reconstructed using an intra block copy (IBC) mode based on the stored reconstructed samples of a reference sub-block of the reconstructed block or the stored reconstructed samples of a reference sub-block of the current block.

INCORPORATION BY REFERENCE

This present application claims the benefit of priority to U.S.Provisional Application No. 62/746,857, “Reference search rangeoptimization for intra picture block compensation” filed on Oct. 17,2018, which is incorporated by reference herein in its entirety.

TECHNICAL FIELD

The present disclosure describes embodiments generally related to videocoding.

BACKGROUND

The background description provided herein is for the purpose ofgenerally presenting the context of the disclosure. Work of thepresently named inventors, to the extent the work is described in thisbackground section, as well as aspects of the description that may nototherwise qualify as prior art at the time of filing, are neitherexpressly nor impliedly admitted as prior art against the presentdisclosure.

Video coding and decoding can be performed using inter-pictureprediction with motion compensation. Uncompressed digital video caninclude a series of pictures, each picture having a spatial dimensionof, for example, 1920×1080 luminance samples and associated chrominancesamples. The series of pictures can have a fixed or variable picturerate (informally also known as frame rate), of, for example 60 picturesper second or 60 Hz. Uncompressed video has significant bitraterequirements. For example, 1080p60 4:2:0 video at 8 bit per sample(1920×1080 luminance sample resolution at 60 Hz frame rate) requiresclose to 1.5 Gbit/s bandwidth. An hour of such video requires more than600 GBytes of storage space.

One purpose of video coding and decoding can be the reduction ofredundancy in the input video signal, through compression. Compressioncan help reduce the aforementioned bandwidth or storage spacerequirements, in some cases by two orders of magnitude or more. Bothlossless and lossy compression, as well as a combination thereof can beemployed. Lossless compression refers to techniques where an exact copyof the original signal can be reconstructed from the compressed originalsignal. When using lossy compression, the reconstructed signal may notbe identical to the original signal, but the distortion between originaland reconstructed signals is small enough to make the reconstructedsignal useful for the intended application. In the case of video, lossycompression is widely employed. The amount of distortion tolerateddepends on the application; for example, users of certain consumerstreaming applications may tolerate higher distortion than users oftelevision distribution applications. The compression ratio achievablecan reflect that: higher allowable/tolerable distortion can yield highercompression ratios.

Motion compensation can be a lossy compression technique and can relateto techniques where a block of sample data from a previouslyreconstructed picture or part thereof (reference picture), after beingspatially shifted in a direction indicated by a motion vector (MVhenceforth), is used for the prediction of a newly reconstructed pictureor picture part. In some cases, the reference picture can be the same asthe picture currently under reconstruction. MVs can have two dimensionsX and Y, or three dimensions, the third being an indication of thereference picture in use (the latter, indirectly, can be a timedimension).

In some video compression techniques, an MV applicable to a certain areaof sample data can be predicted from other MVs, for example from thoserelated to another area of sample data spatially adjacent to the areaunder reconstruction, and preceding that MV in decoding order. Doing socan substantially reduce the amount of data required for coding the MV,thereby removing redundancy and increasing compression. MV predictioncan work effectively, for example, because when coding an input videosignal derived from a camera (known as natural video) there is astatistical likelihood that areas larger than the area to which a singleMV is applicable move in a similar direction and, therefore, can in somecases be predicted using a similar motion vector derived from MVs ofneighboring area. That results in the MV found for a given area to besimilar or the same as the MV predicted from the surrounding MVs, andthat in turn can be represented, after entropy coding, in a smallernumber of bits than what would be used if coding the MV directly. Insome cases, MV prediction can be an example of lossless compression of asignal (namely: the MVs) derived from the original signal (namely: thesample stream). In other cases, MV prediction itself can be lossy, forexample because of rounding errors when calculating a predictor fromseveral surrounding MVs.

Various MV prediction mechanisms are described in H.265/HEVC (ITU-T Rec.H.265, “High Efficiency Video Coding”, December 2016). Out of the manyMV prediction mechanisms that H.265 offers, described here is atechnique henceforth referred to as “spatial merge”.

Referring to FIG. 1, a current block (101) comprises samples that havebeen found by the encoder during the motion search process to bepredictable from a previous block of the same size that has beenspatially shifted. Instead of coding that MV directly, the MV can bederived from metadata associated with one or more reference pictures,for example from the most recent (in decoding order) reference picture,using the MV associated with either one of five surrounding samples,denoted A0, A1, and B0, B1, B2 (102 through 106, respectively). InH.265, the MV prediction can use predictors from the same referencepicture that the neighboring block is using.

SUMMARY

Aspects of the disclosure provide methods and apparatuses for videocoding at a decoder. In an embodiment, a method of video coding at adecoder is provided. In the method, reconstructed samples of areconstructed block of a picture are stored in a first reference samplememory. The first reference sample memory is configured to store atleast one set of a number of luma samples and corresponding chromasamples of the reconstructed block. Further, reconstructed samples of acurrent block of the picture are stored in a second reference samplememory. The second reference sample memory is configured to store onlyone set of the number of luma samples and corresponding chroma samplesof the current block. A current sub-block in the current block isreconstructed using an intra block copy (IBC) mode based on the storedreconstructed samples of a reference sub-block of the reconstructedblock or the stored reconstructed samples of a reference sub-block ofthe current block.

In an embodiment, each of the at least one set of the luma samples ofthe reconstructed block and the one set of the luma samples of thecurrent block includes 64×64 luma samples.

In an embodiment, a maximum size of the first reference sample memory islimited to a size of two sets of the luma samples and the correspondingchroma samples.

In an embodiment, a coding tree unit (CTU) is partitioned into one ormore non-overlapping blocks. The one or more non-overlapping blocksinclude the current block, and the reconstructed block is determinedbased on a location of the current block relative to the CTU and adecoding order of the one or more non-overlapping blocks. In someexamples, when the current block is a top-left block of the CTU, thereconstructed block is determined to be a top-right block of anotherCTU. The other CTU is to the left of the CTU. In some examples, when thecurrent block is a top-right block of the CTU or a bottom-left block ofthe CTU, the reconstructed block is determined to be a top-left block ofthe CTU. In some examples, when the current block is a bottom-rightblock of the CTU, the reconstructed block is determined to be atop-right block of the CTU. In some examples, when the current block isa bottom-left block of the CTU, the reconstructed block is determined tobe a top-right block of the CTU.

In an embodiment, reconstructed samples of another reconstructed blockof the picture are stored in the first reference sample memory. A sizeof the other reconstructed block does not exceed one set of the numberof luma samples and corresponding chroma samples. The current sub-blockin the current block is reconstructed using the IBC mode based on thestored reconstructed samples of the reference sub-block of thereconstructed block, the stored reconstructed samples of a referencesub-block of the other reconstructed block, or the stored reconstructedsamples of the reference sub-block of the current block. In someexamples, when the current block is a top-left block of the CTU, thereconstructed block is determined to be a top-right block and abottom-right block of another CTU. The other CTU is to the left of theCTU.

In an embodiment, a method of video coding at a decoder is provided. Inthe method, reconstructed samples of a current block of a picture arestored in a reference sample memory. A size of the current block doesnot exceed one set of luma samples and corresponding chroma samples. Acurrent sub-block in the current block is reconstructed using an intrablock copy (IBC) mode based on the stored reconstructed samples of areference sub-block of the current block. A maximum size of thereference sample memory is limited to the one set of luma samples andcorresponding chroma samples.

Aspects of the disclosure also provide non-transitory computer-readablestorage mediums storing instructions which when executed by a computercause the computer to perform any of the above methods.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features, the nature, and various advantages of the disclosedsubject matter will be more apparent from the following detaileddescription and the accompanying drawings in which:

FIG. 1 is a schematic illustration of a current block and itssurrounding spatial merge candidates in one example.

FIG. 2 is a schematic illustration of a simplified block diagram of acommunication system (200) in accordance with an embodiment.

FIG. 3 is a schematic illustration of a simplified block diagram of acommunication system (300) in accordance with an embodiment.

FIG. 4 is a schematic illustration of a simplified block diagram of adecoder in accordance with an embodiment.

FIG. 5 is a schematic illustration of a simplified block diagram of anencoder in accordance with an embodiment.

FIG. 6 shows a block diagram of an encoder in accordance with anotherembodiment.

FIG. 7 shows a block diagram of a decoder in accordance with anotherembodiment.

FIG. 8 is a schematic illustration of a current block in a currentpicture to be coded using intra block copy (IBC) in accordance with anembodiment.

FIG. 9 describes an example of IBC-based compensation with referencesamples stored in two sets of memory blocks in accordance with anembodiment.

FIG. 10 describes an example of IBC-based compensation with referencesamples stored in three sets of memory blocks in accordance with anembodiment.

FIG. 11 describes an example of IBC-based compensation with referencesamples stored in one memory block in accordance with an embodiment.

FIG. 12 shows a flow chart outlining a decoding process (S1200)according to an embodiment of the disclosure.

FIG. 13 shows a flow chart outlining a decoding process (S1300)according to an embodiment of the disclosure.

FIG. 14 is a schematic illustration of a computer system in accordancewith an embodiment.

DETAILED DESCRIPTION OF EMBODIMENTS

I. Video Coding Encoder and Decoder

FIG. 2 illustrates a simplified block diagram of a communication system(200) according to an embodiment of the present disclosure. Thecommunication system (200) includes a plurality of terminal devices thatcan communicate with each other, via, for example, a network (250). Forexample, the communication system (200) includes a first pair ofterminal devices (210) and (220) interconnected via the network (250).In the FIG. 2 example, the first pair of terminal devices (210) and(220) performs unidirectional transmission of data. For example, theterminal device (210) may code video data (e.g., a stream of videopictures that are captured by the terminal device (210)) fortransmission to the other terminal device (220) via the network (250).The encoded video data can be transmitted in the form of one or morecoded video bitstreams. The terminal device (220) may receive the codedvideo data from the network (250), decode the coded video data torecover the video pictures and display video pictures according to therecovered video data. Unidirectional data transmission may be common inmedia serving applications and the like.

In another example, the communication system (200) includes a secondpair of terminal devices (230) and (240) that performs bidirectionaltransmission of coded video data that may occur, for example, duringvideoconferencing. For bidirectional transmission of data, in anexample, each terminal device of the terminal devices (230) and (240)may code video data (e.g., a stream of video pictures that are capturedby the terminal device) for transmission to the other terminal device ofthe terminal devices (230) and (240) via the network (250). Eachterminal device of the terminal devices (230) and (240) also may receivethe coded video data transmitted by the other terminal device of theterminal devices (230) and (240), and may decode the coded video data torecover the video pictures and may display video pictures at anaccessible display device according to the recovered video data.

In the FIG. 2 example, the terminal devices (210), (220), (230) and(240) may be illustrated as servers, personal computers, and smartphones, but the principles of the present disclosure may be not solimited. Embodiments of the present disclosure find application withlaptop computers, tablet computers, media players, and/or dedicatedvideo conferencing equipment. The network (250) represents any number ofnetworks that convey coded video data among the terminal devices (210),(220), (230), and (240), including for example wireline (wired) and/orwireless communication networks. The communication network (250) mayexchange data in circuit-switched and/or packet-switched channels.Representative networks include telecommunications networks, local areanetworks, wide area networks, and/or the Internet. For the purposes ofthe present discussion, the architecture and topology of the network(250) may be immaterial to the operation of the present disclosureunless explained herein below.

FIG. 3 illustrates, as an example for an application for the disclosedsubject matter, the placement of a video encoder and a video decoder ina streaming environment. The disclosed subject matter can be equallyapplicable to other video enabled applications, including, for example,video conferencing, digital TV, storing of compressed video on digitalmedia including CD, DVD, memory stick and the like, and so on.

A streaming system may include a capture subsystem (313), that caninclude a video source (301), for example a digital camera, creating forexample a stream of video pictures (302) that are uncompressed. In anexample, the stream of video pictures (302) includes samples that aretaken by the digital camera. The stream of video pictures (302),depicted as a bold line to emphasize a high data volume when compared toencoded video data (304) (or coded video bitstreams), can be processedby an electronic device (320) that includes a video encoder (303)coupled to the video source (301). The video encoder (303) can includehardware, software, or a combination thereof to enable or implementaspects of the disclosed subject matter as described in more detailbelow. The encoded video data (304) (or encoded video bitstream (304)),depicted as a thin line to emphasize the lower data volume when comparedto the stream of video pictures (302), can be stored on a streamingserver (305) for future use. One or more streaming client subsystems,such as client subsystems (306) and (308) in FIG. 3 can access thestreaming server (305) to retrieve copies (307) and (309) of the encodedvideo data (304). A client subsystem (306) can include a video decoder(310), for example, in an electronic device (330). The video decoder(310) decodes the incoming copy (307) of the encoded video data andcreates an outgoing stream of video pictures (311) that can be renderedon a display (312) (e.g., display screen) or other rendering device (notdepicted). In some streaming systems, the encoded video data (304),(307), and (309) (e.g., video bitstreams) can be encoded according tocertain video coding/compression standards. Examples of those standardsinclude ITU-T Recommendation H.265. In an example, a video codingstandard under development is informally known as Versatile Video Coding(VVC). The disclosed subject matter may be used in the context of VVC.

It is noted that the electronic devices (320) and (330) can includeother components (not shown). For example, the electronic device (320)can include a video decoder (not shown) and the electronic device (330)can include a video encoder (not shown) as well.

FIG. 4 shows a block diagram of a video decoder (410) according to anembodiment of the present disclosure. The video decoder (410) can beincluded in an electronic device (430). The electronic device (430) caninclude a receiver (431) (e.g., receiving circuitry). The video decoder(410) can be used in the place of the video decoder (310) in the FIG. 3example.

The receiver (431) may receive one or more coded video sequences to bedecoded by the video decoder (410); in the same or another embodiment,one coded video sequence at a time, where the decoding of each codedvideo sequence is independent from other coded video sequences. Thecoded video sequence may be received from a channel (401), which may bea hardware/software link to a storage device which stores the encodedvideo data. The receiver (431) may receive the encoded video data withother data, for example, coded audio data and/or ancillary data streams,that may be forwarded to their respective using entities (not depicted).The receiver (431) may separate the coded video sequence from the otherdata. To combat network jitter, a buffer memory (415) may be coupled inbetween the receiver (431) and an entropy decoder/parser (420) (“parser(420)” henceforth). In certain applications, the buffer memory (415) ispart of the video decoder (410). In others, it can be outside of thevideo decoder (410) (not depicted). In still others, there can be abuffer memory (not depicted) outside of the video decoder (410), forexample to combat network jitter, and in addition another buffer memory(415) inside the video decoder (410), for example to handle playouttiming. When the receiver (431) is receiving data from a store/forwarddevice of sufficient bandwidth and controllability, or from anisosynchronous network, the buffer memory (415) may not be needed, orcan be small. For use on best effort packet networks such as theInternet, the buffer memory (415) may be required, can be comparativelylarge and can be advantageously of adaptive size, and may at leastpartially be implemented in an operating system or similar elements (notdepicted) outside of the video decoder (410).

The video decoder (410) may include the parser (420) to reconstructsymbols (421) from the coded video sequence. Categories of those symbolsinclude information used to manage operation of the video decoder (410),and potentially information to control a rendering device such as arender device (412) (e.g., a display screen) that is not an integralpart of the electronic device (430) but can be coupled to the electronicdevice (430), as was shown in FIG. 4. The control information for therendering device(s) may be in the form of Supplemental EnhancementInformation (SEI messages) or Video Usability Information (VUI)parameter set fragments (not depicted). The parser (420) mayparse/entropy-decode the coded video sequence that is received. Thecoding of the coded video sequence can be in accordance with a videocoding technology or standard, and can follow various principles,including variable length coding, Huffman coding, arithmetic coding withor without context sensitivity, and so forth. The parser (420) mayextract from the coded video sequence, a set of subgroup parameters forat least one of the subgroups of pixels in the video decoder, based uponat least one parameter corresponding to the group. Subgroups can includeGroups of Pictures (GOPs), pictures, tiles, slices, macroblocks, CodingUnits (CUs), blocks, Transform Units (TUs), Prediction Units (PUs), andso forth. The parser (420) may also extract from the coded videosequence information such as transform coefficients, quantizer parametervalues, motion vectors, and so forth.

The parser (420) may perform an entropy decoding/parsing operation onthe video sequence received from the buffer memory (415), so as tocreate symbols (421).

Reconstruction of the symbols (421) can involve multiple different unitsdepending on the type of the coded video picture or parts thereof (suchas: inter and intra picture, inter and intra block), and other factors.Which units are involved, and how, can be controlled by the subgroupcontrol information that was parsed from the coded video sequence by theparser (420). The flow of such subgroup control information between theparser (420) and the multiple units below is not depicted for clarity.

Beyond the functional blocks already mentioned, the video decoder (410)can be conceptually subdivided into a number of functional units asdescribed below. In a practical implementation operating undercommercial constraints, many of these units interact closely with eachother and can, at least partly, be integrated into each other. However,for the purpose of describing the disclosed subject matter, theconceptual subdivision into the functional units below is appropriate.

A first unit is the scaler/inverse transform unit (451). Thescaler/inverse transform unit (451) receives a quantized transformcoefficient as well as control information, including which transform touse, block size, quantization factor, quantization scaling matrices,etc. as symbol(s) (421) from the parser (420). The scaler/inversetransform unit (451) can output blocks comprising sample values, thatcan be input into aggregator (455).

In some cases, the output samples of the scaler/inverse transform (451)can pertain to an intra coded block; that is: a block that is not usingpredictive information from previously reconstructed pictures, but canuse predictive information from previously reconstructed parts of thecurrent picture. Such predictive information can be provided by an intrapicture prediction unit (452). In some cases, the intra pictureprediction unit (452) generates a block of the same size and shape ofthe block under reconstruction, using surrounding already reconstructedinformation fetched from the current picture buffer (458). The currentpicture buffer (458) buffers, for example, partly reconstructed currentpicture and/or fully reconstructed current picture. The aggregator(455), in some cases, adds, on a per sample basis, the predictioninformation the intra prediction unit (452) has generated to the outputsample information as provided by the scaler/inverse transform unit(451).

In other cases, the output samples of the scaler/inverse transform unit(451) can pertain to an inter coded, and potentially motion compensatedblock. In such a case, a motion compensation prediction unit (453) canaccess reference picture memory (457) to fetch samples used forprediction. After motion compensating the fetched samples in accordancewith the symbols (421) pertaining to the block, these samples can beadded by the aggregator (455) to the output of the scaler/inversetransform unit (451) (in this case called the residual samples orresidual signal) so as to generate output sample information. Theaddresses within the reference picture memory (457) from where themotion compensation prediction unit (453) fetches prediction samples canbe controlled by motion vectors, available to the motion compensationprediction unit (453) in the form of symbols (421) that can have, forexample X, Y, and reference picture components. Motion compensation alsocan include interpolation of sample values as fetched from the referencepicture memory (457) when sub-sample exact motion vectors are in use,motion vector prediction mechanisms, and so forth.

The output samples of the aggregator (455) can be subject to variousloop filtering techniques in the loop filter unit (456). Videocompression technologies can include in-loop filter technologies thatare controlled by parameters included in the coded video sequence (alsoreferred to as coded video bitstream) and made available to the loopfilter unit (456) as symbols (421) from the parser (420), but can alsobe responsive to meta-information obtained during the decoding ofprevious (in decoding order) parts of the coded picture or coded videosequence, as well as responsive to previously reconstructed andloop-filtered sample values.

The output of the loop filter unit (456) can be a sample stream that canbe output to the render device (412) as well as stored in the referencepicture memory (457) for use in future inter-picture prediction.

Certain coded pictures, once fully reconstructed, can be used asreference pictures for future prediction. For example, once a codedpicture corresponding to a current picture is fully reconstructed andthe coded picture has been identified as a reference picture (by, forexample, the parser (420)), the current picture buffer (458) can becomea part of the reference picture memory (457), and a fresh currentpicture buffer can be reallocated before commencing the reconstructionof the following coded picture.

The video decoder (410) may perform decoding operations according to apredetermined video compression technology in a standard, such as ITU-TRec. H.265. The coded video sequence may conform to a syntax specifiedby the video compression technology or standard being used, in the sensethat the coded video sequence adheres to both the syntax of the videocompression technology or standard and the profiles as documented in thevideo compression technology or standard. Specifically, a profile canselect certain tools as the only tools available for use under thatprofile from all the tools available in the video compression technologyor standard. Also necessary for compliance can be that the complexity ofthe coded video sequence is within bounds as defined by the level of thevideo compression technology or standard. In some cases, levels restrictthe maximum picture size, maximum frame rate, maximum reconstructionsample rate (measured in, for example megasamples per second), maximumreference picture size, and so on. Limits set by levels can, in somecases, be further restricted through Hypothetical Reference Decoder(HRD) specifications and metadata for HRD buffer management signaled inthe coded video sequence.

In an embodiment, the receiver (431) may receive additional (redundant)data with the encoded video. The additional data may be included as partof the coded video sequence(s). The additional data may be used by thevideo decoder (410) to properly decode the data and/or to moreaccurately reconstruct the original video data. Additional data can bein the form of, for example, temporal, spatial, or signal noise ratio(SNR) enhancement layers, redundant slices, redundant pictures, forwarderror correction codes, and so on.

FIG. 5 shows a block diagram of a video encoder (503) according to anembodiment of the present disclosure. The video encoder (503) isincluded in an electronic device (520). The electronic device (520)includes a transmitter (540) (e.g., transmitting circuitry). The videoencoder (503) can be used in the place of the video encoder (303) in theFIG. 3 example.

The video encoder (503) may receive video samples from a video source(501) (that is not part of the electronic device (520) in the FIG. 5example) that may capture video image(s) to be coded by the videoencoder (503). In another example, the video source (501) is a part ofthe electronic device (520).

The video source (501) may provide the source video sequence to be codedby the video encoder (503) in the form of a digital video sample streamthat can be of any suitable bit depth (for example: 8 bit, 10 bit, 12bit, . . . ), any colorspace (for example, BT.601 Y CrCB, RGB, . . . ),and any suitable sampling structure (for example Y CrCb 4:2:0, Y CrCb4:4:4). In a media serving system, the video source (501) may be astorage device storing previously prepared video. In a videoconferencingsystem, the video source (501) may be a camera that captures local imageinformation as a video sequence. Video data may be provided as aplurality of individual pictures that impart motion when viewed insequence. The pictures themselves may be organized as a spatial array ofpixels, wherein each pixel can comprise one or more samples depending onthe sampling structure, color space, etc. in use. A person skilled inthe art can readily understand the relationship between pixels andsamples. The description below focuses on samples.

According to an embodiment, the video encoder (503) may code andcompress the pictures of the source video sequence into a coded videosequence (543) in real time or under any other time constraints asrequired by the application. Enforcing appropriate coding speed is onefunction of a controller (550). In some embodiments, the controller(550) controls other functional units as described below and isfunctionally coupled to the other functional units. The coupling is notdepicted for clarity. Parameters set by the controller (550) can includerate control related parameters (picture skip, quantizer, lambda valueof rate-distortion optimization techniques, . . . ), picture size, groupof pictures (GOP) layout, maximum motion vector search range, and soforth. The controller (550) can be configured to have other suitablefunctions that pertain to the video encoder (503) optimized for acertain system design.

In some embodiments, the video encoder (503) is configured to operate ina coding loop. As an oversimplified description, in an example, thecoding loop can include a source coder (530) (e.g., responsible forcreating symbols, such as a symbol stream, based on an input picture tobe coded, and a reference picture(s)), and a (local) decoder (533)embedded in the video encoder (503). The decoder (533) reconstructs thesymbols to create the sample data in a similar manner as a (remote)decoder also would create (as any compression between symbols and codedvideo bitstream is lossless in the video compression technologiesconsidered in the disclosed subject matter). The reconstructed samplestream (sample data) is input to the reference picture memory (534). Asthe decoding of a symbol stream leads to bit-exact results independentof decoder location (local or remote), the content in the referencepicture memory (534) is also bit exact between the local encoder andremote encoder. In other words, the prediction part of an encoder “sees”as reference picture samples exactly the same sample values as a decoderwould “see” when using prediction during decoding. This fundamentalprinciple of reference picture synchronicity (and resulting drift, ifsynchronicity cannot be maintained, for example because of channelerrors) is used in some related arts as well.

The operation of the “local” decoder (533) can be the same as of a“remote” decoder, such as the video decoder (410), which has alreadybeen described in detail above in conjunction with FIG. 4. Brieflyreferring also to FIG. 4, however, as symbols are available andencoding/decoding of symbols to a coded video sequence by an entropycoder (545) and the parser (420) can be lossless, the entropy decodingparts of the video decoder (410), including the buffer memory (415), andparser (420) may not be fully implemented in the local decoder (533).

An observation that can be made at this point is that any decodertechnology except the parsing/entropy decoding that is present in adecoder also necessarily needs to be present, in substantially identicalfunctional form, in a corresponding encoder. For this reason, thedisclosed subject matter focuses on decoder operation. The descriptionof encoder technologies can be abbreviated as they are the inverse ofthe comprehensively described decoder technologies. Only in certainareas a more detail description is required and provided below.

During operation, in some examples, the source coder (530) may performmotion compensated predictive coding, which codes an input picturepredictively with reference to one or more previously-coded picture fromthe video sequence that were designated as “reference pictures”. In thismanner, the coding engine (532) codes differences between pixel blocksof an input picture and pixel blocks of reference picture(s) that may beselected as prediction reference(s) to the input picture.

The local video decoder (533) may decode coded video data of picturesthat may be designated as reference pictures, based on symbols createdby the source coder (530). Operations of the coding engine (532) mayadvantageously be lossy processes. When the coded video data may bedecoded at a video decoder (not shown in FIG. 5), the reconstructedvideo sequence typically may be a replica of the source video sequencewith some errors. The local video decoder (533) replicates decodingprocesses that may be performed by the video decoder on referencepictures and may cause reconstructed reference pictures to be stored inthe reference picture cache (534). In this manner, the video encoder(503) may store copies of reconstructed reference pictures locally thathave common content as the reconstructed reference pictures that will beobtained by a far-end video decoder (absent transmission errors).

The predictor (535) may perform prediction searches for the codingengine (532). That is, for a new picture to be coded, the predictor(535) may search the reference picture memory (534) for sample data (ascandidate reference pixel blocks) or certain metadata such as referencepicture motion vectors, block shapes, and so on, that may serve as anappropriate prediction reference for the new pictures. The predictor(535) may operate on a sample block-by-pixel block basis to findappropriate prediction references. In some cases, as determined bysearch results obtained by the predictor (535), an input picture mayhave prediction references drawn from multiple reference pictures storedin the reference picture memory (534).

The controller (550) may manage coding operations of the source coder(530), including, for example, setting of parameters and subgroupparameters used for encoding the video data.

Output of all aforementioned functional units may be subjected toentropy coding in the entropy coder (545). The entropy coder (545)translates the symbols as generated by the various functional units intoa coded video sequence, by lossless compressing the symbols according totechnologies such as Huffman coding, variable length coding, arithmeticcoding, and so forth.

The transmitter (540) may buffer the coded video sequence(s) as createdby the entropy coder (545) to prepare for transmission via acommunication channel (560), which may be a hardware/software link to astorage device which would store the encoded video data. The transmitter(540) may merge coded video data from the video coder (503) with otherdata to be transmitted, for example, coded audio data and/or ancillarydata streams (sources not shown).

The controller (550) may manage operation of the video encoder (503).During coding, the controller (550) may assign to each coded picture acertain coded picture type, which may affect the coding techniques thatmay be applied to the respective picture. For example, pictures oftenmay be assigned as one of the following picture types:

An Intra Picture (I picture) may be one that may be coded and decodedwithout using any other picture in the sequence as a source ofprediction. Some video codecs allow for different types of intrapictures, including, for example Independent Decoder Refresh (“IDR”)Pictures. A person skilled in the art is aware of those variants of Ipictures and their respective applications and features.

A predictive picture (P picture) may be one that may be coded anddecoded using intra prediction or inter prediction using at most onemotion vector and reference index to predict the sample values of eachblock.

A bi-directionally predictive picture (B Picture) may be one that may becoded and decoded using intra prediction or inter prediction using atmost two motion vectors and reference indices to predict the samplevalues of each block. Similarly, multiple-predictive pictures can usemore than two reference pictures and associated metadata for thereconstruction of a single block.

Source pictures commonly may be subdivided spatially into a plurality ofsample blocks (for example, blocks of 4×4, 8×8, 4×8, or 16×16 sampleseach) and coded on a block-by-block basis. Blocks may be codedpredictively with reference to other (already coded) blocks asdetermined by the coding assignment applied to the blocks' respectivepictures. For example, blocks of I pictures may be codednon-predictively or they may be coded predictively with reference toalready coded blocks of the same picture (spatial prediction or intraprediction). Pixel blocks of P pictures may be coded predictively, viaspatial prediction or via temporal prediction with reference to onepreviously coded reference picture. Blocks of B pictures may be codedpredictively, via spatial prediction or via temporal prediction withreference to one or two previously coded reference pictures.

The video encoder (503) may perform coding operations according to apredetermined video coding technology or standard, such as ITU-T Rec.H.265. In its operation, the video encoder (503) may perform variouscompression operations, including predictive coding operations thatexploit temporal and spatial redundancies in the input video sequence.The coded video data, therefore, may conform to a syntax specified bythe video coding technology or standard being used.

In an embodiment, the transmitter (540) may transmit additional datawith the encoded video. The source coder (530) may include such data aspart of the coded video sequence. Additional data may comprisetemporal/spatial/SNR enhancement layers, other forms of redundant datasuch as redundant pictures and slices, SEI messages, VUI parameter setfragments, and so on.

A video may be captured as a plurality of source pictures (videopictures) in a temporal sequence. Intra-picture prediction (oftenabbreviated to intra prediction) makes use of spatial correlation in agiven picture, and inter-picture prediction makes uses of the (temporalor other) correlation between the pictures. In an example, a specificpicture under encoding/decoding, which is referred to as a currentpicture, is partitioned into blocks. When a block in the current pictureis similar to a reference block in a previously coded and still bufferedreference picture in the video, the block in the current picture can becoded by a vector that is referred to as a motion vector. The motionvector points to the reference block in the reference picture, and canhave a third dimension identifying the reference picture, in casemultiple reference pictures are in use.

In some embodiments, a bi-prediction technique can be used in theinter-picture prediction. According to the bi-prediction technique, tworeference pictures, such as a first reference picture and a secondreference picture that are both prior in decoding order to the currentpicture in the video (but may be in the past and future, respectively,in display order) are used. A block in the current picture can be codedby a first motion vector that points to a first reference block in thefirst reference picture, and a second motion vector that points to asecond reference block in the second reference picture. The block can bepredicted by a combination of the first reference block and the secondreference block.

Further, a merge mode technique can be used in the inter-pictureprediction to improve coding efficiency.

According to some embodiments of the disclosure, predictions, such asinter-picture predictions and intra-picture predictions are performed inthe unit of blocks. For example, according to the HEVC standard, apicture in a sequence of video pictures is partitioned into coding treeunits (CTU) for compression, the CTUs in a picture have the same size,such as 64×64 pixels, 32×32 pixels, or 16×16 pixels. In general, a CTUincludes three coding tree blocks (CTBs), which are one luma CTB and twochroma CTBs. Each CTU can be recursively quadtree split into one ormultiple coding units (CUs). For example, a CTU of 64×64 pixels can besplit into one CU of 64×64 pixels, or 4 CUs of 32×32 pixels, or 16 CUsof 16×16 pixels. In an example, each CU is analyzed to determine aprediction type for the CU, such as an inter prediction type or an intraprediction type. The CU is split into one or more prediction units (PUs)depending on the temporal and/or spatial predictability. Generally, eachPU includes a luma prediction block (PB), and two chroma PBs. In anembodiment, a prediction operation in coding (encoding/decoding) isperformed in the unit of a prediction block. Using a luma predictionblock as an example of a prediction block, the prediction block includesa matrix of values (e.g., luma values) for pixels, such as 8×8 pixels,16×16 pixels, 8×16 pixels, 16×8 pixels, and the like.

FIG. 6 shows a diagram of a video encoder (603) according to anotherembodiment of the disclosure. The video encoder (603) is configured toreceive a processing block (e.g., a prediction block) of sample valueswithin a current video picture in a sequence of video pictures, andencode the processing block into a coded picture that is part of a codedvideo sequence. In an example, the video encoder (603) is used in theplace of the video encoder (303) in the FIG. 3 example.

In an HEVC example, the video encoder (603) receives a matrix of samplevalues for a processing block, such as a prediction block of 8×8samples, and the like. The video encoder (603) determines whether theprocessing block is best coded using intra mode, inter mode, orbi-prediction mode using, for example, rate-distortion optimization.When the processing block is to be coded in intra mode, the videoencoder (603) may use an intra prediction technique to encode theprocessing block into the coded picture; and when the processing blockis to be coded in inter mode or bi-prediction mode, the video encoder(603) may use an inter prediction or bi-prediction technique,respectively, to encode the processing block into the coded picture. Incertain video coding technologies, merge mode can be an inter pictureprediction submode where the motion vector is derived from one or moremotion vector predictors without the benefit of a coded motion vectorcomponent outside the predictors. In certain other video codingtechnologies, a motion vector component applicable to the subject blockmay be present. In an example, the video encoder (603) includes othercomponents, such as a mode decision module (not shown) to determine themode of the processing blocks.

In the FIG. 6 example, the video encoder (603) includes the interencoder (630), an intra encoder (622), a residue calculator (623), aswitch (626), a residue encoder (624), a general controller (621), andan entropy encoder (625) coupled together as shown in FIG. 6.

The inter encoder (630) is configured to receive the samples of thecurrent block (e.g., a processing block), compare the block to one ormore reference blocks in reference pictures (e.g., blocks in previouspictures and later pictures), generate inter prediction information(e.g., description of redundant information according to inter encodingtechnique, motion vectors, merge mode information), and calculate interprediction results (e.g., predicted block) based on the inter predictioninformation using any suitable technique. In some examples, thereference pictures are decoded reference pictures that are decoded basedon the encoded video information.

The intra encoder (622) is configured to receive the samples of thecurrent block (e.g., a processing block), in some cases compare theblock to blocks already coded in the same picture, generate quantizedcoefficients after transform, and in some cases also intra predictioninformation (e.g., an intra prediction direction information accordingto one or more intra encoding techniques). In an example, the intraencoder (622) also calculates intra prediction results (e.g., predictedblock) based on the intra prediction information and reference blocks inthe same picture.

The general controller (621) is configured to determine general controldata and control other components of the video encoder (603) based onthe general control data. In an example, the general controller (621)determines the mode of the block, and provides a control signal to theswitch (626) based on the mode. For example, when the mode is the intramode, the general controller (621) controls the switch (626) to selectthe intra mode result for use by the residue calculator (623), andcontrols the entropy encoder (625) to select the intra predictioninformation and include the intra prediction information in thebitstream; and when the mode is the inter mode, the general controller(621) controls the switch (626) to select the inter prediction resultfor use by the residue calculator (623), and controls the entropyencoder (625) to select the inter prediction information and include theinter prediction information in the bitstream.

The residue calculator (623) is configured to calculate a difference(residue data) between the received block and prediction resultsselected from the intra encoder (622) or the inter encoder (630). Theresidue encoder (624) is configured to operate based on the residue datato encode the residue data to generate the transform coefficients. In anexample, the residue encoder (624) is configured to convert the residuedata from a spatial domain to a frequency domain, and generate thetransform coefficients. The transform coefficients are then subject toquantization processing to obtain quantized transform coefficients. Invarious embodiments, the video encoder (603) also includes a residuedecoder (628). The residue decoder (628) is configured to performinverse-transform, and generate the decoded residue data. The decodedresidue data can be suitably used by the intra encoder (622) and theinter encoder (630). For example, the inter encoder (630) can generatedecoded blocks based on the decoded residue data and inter predictioninformation, and the intra encoder (622) can generate decoded blocksbased on the decoded residue data and the intra prediction information.The decoded blocks are suitably processed to generate decoded picturesand the decoded pictures can be buffered in a memory circuit (not shown)and used as reference pictures in some examples.

The entropy encoder (625) is configured to format the bitstream toinclude the encoded block. The entropy encoder (625) is configured toinclude various information according to a suitable standard, such asthe HEVC standard. In an example, the entropy encoder (625) isconfigured to include the general control data, the selected predictioninformation (e.g., intra prediction information or inter predictioninformation), the residue information, and other suitable information inthe bitstream. Note that, according to the disclosed subject matter,when coding a block in the merge submode of either inter mode orbi-prediction mode, there is no residue information.

FIG. 7 shows a diagram of a video decoder (710) according to anotherembodiment of the disclosure. The video decoder (710) is configured toreceive coded pictures that are part of a coded video sequence, anddecode the coded pictures to generate reconstructed pictures. In anexample, the video decoder (710) is used in the place of the videodecoder (310) in the FIG. 3 example.

In the FIG. 7 example, the video decoder (710) includes an entropydecoder (771), an inter decoder (780), a residue decoder (773), areconstruction module (774), and an intra decoder (772) coupled togetheras shown in FIG. 7.

The entropy decoder (771) can be configured to reconstruct, from thecoded picture, certain symbols that represent the syntax elements ofwhich the coded picture is made up. Such symbols can include, forexample, the mode in which a block is coded (such as, for example, intramode, inter mode, bi-predicted mode, the latter two in merge submode oranother submode), prediction information (such as, for example, intraprediction information or inter prediction information) that canidentify certain sample or metadata that is used for prediction by theintra decoder (772) or the inter decoder (780), respectively, residualinformation in the form of, for example, quantized transformcoefficients, and the like. In an example, when the prediction mode isinter or bi-predicted mode, the inter prediction information is providedto the inter decoder (780); and when the prediction type is the intraprediction type, the intra prediction information is provided to theintra decoder (772). The residual information can be subject to inversequantization and is provided to the residue decoder (773).

The inter decoder (780) is configured to receive the inter predictioninformation, and generate inter prediction results based on the interprediction information.

The intra decoder (772) is configured to receive the intra predictioninformation, and generate prediction results based on the intraprediction information.

The residue decoder (773) is configured to perform inverse quantizationto extract de-quantized transform coefficients, and process thede-quantized transform coefficients to convert the residual from thefrequency domain to the spatial domain. The residue decoder (773) mayalso require certain control information (to include the QuantizerParameter (QP)), and that information may be provided by the entropydecoder (771) (data path not depicted as this may be low volume controlinformation only).

The reconstruction module (774) is configured to combine, in the spatialdomain, the residual as output by the residue decoder (773) and theprediction results (as output by the inter or intra prediction modulesas the case may be) to form a reconstructed block, that may be part ofthe reconstructed picture, which in turn may be part of thereconstructed video. It is noted that other suitable operations, such asa deblocking operation and the like, can be performed to improve thevisual quality.

It is noted that the video encoders (303), (503), and (603), and thevideo decoders (310), (410), and (710) can be implemented using anysuitable technique. In an embodiment, the video encoders (303), (503),and (603), and the video decoders (310), (410), and (710) can beimplemented using one or more integrated circuits. In anotherembodiment, the video encoders (303), (503), and (503), and the videodecoders (310), (410), and (710) can be implemented using one or moreprocessors that execute software instructions.

II. Reference Search Range Optimization of Intra Block Copy

A. Intra Block Copy

A block can be coded using a reference block from a different or samepicture. A block based compensation using a reference block from adifferent picture can be referred to as motion compensation. Block basedcompensation using a reference block from a previously reconstructedarea within the same picture can be referred to as intra picture blockcompensation, current picture referencing (CPR), or intra block copy(IBC). A displacement vector that indicates the offset between thecurrent block and the reference block can be referred to as a blockvector (BV). Different from a motion vector in motion compensation,which can be any value (positive or negative, at either x or ydirection), a BV is subject to constraints to ensure that the referenceblock has already been reconstructed and the reconstructed samplesthereof are available. In some embodiments, in view of parallelprocessing constraints, a reference area that is beyond a tile boundaryor wavefront ladder shape boundary is excluded.

The coding of a BV can be either explicit or implicit. In the explicitmode, the difference between a BV and its predictor can be signaled in amanner similar to an Advanced Motion Vector Prediction (AMVP) mode ininter coding. In the implicit mode, the BV can be recovered from apredictor, for example in a similar way as a motion vector in mergemode. The resolution of a BV, in some implementations, is set to integerpositions or, in some examples, fractional positions.

The use of IBC at the block level can be signaled using a block levelflag (or IBC flag). In some examples, this flag can be signaled when thecurrent block is not coded in merge mode. In some examples, this flagcan be signaled by a reference index approach, for example, by treatingthe current decoded picture as a reference picture. In HEVC ScreenContent Coding (HEVC SCC), such a reference picture is placed in thelast position of the list. This special reference picture is alsomanaged together with other temporal reference pictures in the DecodedPicture Buffer (DPB).

While an embodiment of intra block copy is used as an example in thepresent disclosure, the embodiments of the present disclosure can beapplied to variations for intra block copy. The variations for intrablock copy include, for example, flipped intra block copy where thereference block is flipped horizontally or vertically before being usedto predict current block, or line based intra block copy where eachcompensation unit inside an M×N coding block is an M×1 or 1×N line.

FIG. 8 is a schematic illustration of a current block (810) in a currentpicture (800) to be coded using intra block copy (IBC) in accordancewith an embodiment. In FIG. 8, an example of using IBC is shown wherethe current picture (800) includes 15 blocks arranged into 3 rows and 5columns. In some examples, each block corresponds to a coding tree unit(CTU). The current block (810) includes a sub-block (812) (e.g., acoding block in the CTU) that has a block vector (822) pointing to areference sub-block (832) in the current picture (800).

The reconstructed samples of the current picture can be stored in amemory or memory block (e.g., a dedicated or designated memory orportion of memory). In consideration of implementation cost, thereference area where the reconstructed samples for reference blocksremain available may not be as large as an entire frame, depending on amemory size of the dedicated memory. Therefore, for a current sub-blockusing IBC, in some examples, an IBC reference sub-block may be limitedto only certain neighboring areas, but not the entire picture.

In one example, the memory size is limited to a size of one CTU, whichmeans the IBC mode can only be used when the reference block is withinthe same CTU as the current block. In another example, the memory sizeis limited to a size of two CTUs, which means the IBC mode can only beused when the reference block is either within the current CTU, or theCTU to the left of current CTU. When the reference block is outside theconstrained reference area (i.e., designated local area), even if it hasbeen reconstructed, the reference samples cannot be used for intrapicture block compensation.

With the constrained reference area, the efficiency of IBC is limited.There is a need to further improve the efficiency of IBC with theconstrained reference area.

B. Block Partitioning in VVC

In the current VVC standard, a picture may be divided into an array ofnon-overlapped CTUs. The size of a CTU may be set to be 128×128 lumasamples and the corresponding chroma samples depending on the colorformat. A CTU can be split into CUs using one or a combination of thefollowing tree splitting methods.

For example, a CTU can be split into CUs using Quaternary-Tree (QT)split as in HEVC. This splitting method is the same as in HEVC. That is,each parent block is split in half in both horizontal and verticaldirections. The resulting four smaller partitions are in the same aspectratio as its parent block. In VVC, a CTU is firstly split by QTrecursively. Each QT leaf node (in square shape) can be further splitrecursively using the multi-type (Binary-Tree and Ternary-Tree) tree asdescribed below. The Binary-Tree (BT) split refers to dividing theparent block in half in either horizontal or vertical direction. Theresulting two smaller partitions are half in size as compared to theparent block. The Ternary-Tree (TT) split refers to dividing the parentblock in three parts in either horizontal or vertical direction. Themiddle part of the three is twice as large as the other two parts. Theresulting three smaller partitions are ¼, ½ and ¼ in size respectivelyas compared to the parent block.

The partition of a parent block may be constrained such that at a128×128 level, the following partitioning results for partitioning theparent block are allowed: 128×128, two 128×64, two 64×128, and four64×64. The partition of the parent block may be further constrained suchthat at a 128×64 or 64×128 level, the TT splits at either a horizontalor vertical direction are not allowed. In addition, if there is anyfurther split, the child blocks may be constrained to two 64×64 blocks.

C. Reference Sample Memory

A memory that stores reference samples of previously coded CUs forfuture intra block copy reference may be referred to as a referencesample memory. The reference sample memory can be a dedicated ordesignated memory, as described above.

According to some embodiments of the present disclosure, methods areproposed to improve IBC performance under certain reference areaconstraints. More specifically, the size of a reference sample memorymay be constrained. In the following discussion, the size of thereference sample memory may be fixed to be one set of 64×64 luma samples(together with corresponding chroma samples), two sets of 64×64 lumasamples (together with corresponding chroma samples), three sets of64×64 luma samples (together with corresponding chroma samples), oranother suitable memory size. In one example, the size of the referencesample memory is a size of one CTU, such as one previously coded CTU orone left CTU. In another example, the size of the reference samplememory is the size of two CTUs, such as two previously coded CTUs or twoleft CTUs, or one current CTU together with one left CTU. In someembodiments, each CTU requires a memory size for storing 128×128 lumasamples, together with corresponding chroma samples. When a referenceblock is outside the stored, reconstructed areas, the reference blockcannot be used for IBC.

Embodiments of the present disclosure include methods for utilizing theone or more 64×64 sized reference sample memory blocks to optimize thesearch range of IBC.

The size of an IBC coded block can be as large as any regular intercoded block in general. In some embodiments of the present disclosure,in order to utilize the reference sample memory more efficiently, thesize of an IBC coded block is limited to, for example, 64 luma samplesat either width or height edge and chroma samples with correspondingsize constraints, depending on the color format. The color can be, forexample, in 4:2:0 format and the size of a chroma block in IBC mode maynot be larger than 32 samples on each side. In another example, lowerlimits, such as 32 luma samples on each side can be used as the size ofthe IBC coded block. In the following discussion of the presentdisclosure, it is assumed that the maximum IBC coded block size is 64×64luma samples and corresponding chroma samples. The size of thecorresponding chroma samples may depend on the color format, asdescribed above.

I. Reference Sample Memory with Two Sets of 64×64 Luma Samples

When the maximum size of the reference sample memory that can be used tostore intra block copy reference samples is two sets of 64×64 lumasamples, the following descriptions provide a few methods to efficientlyutilize this memory size to perform IBC.

For example, one 64×64 reference sample memory block is used to storesamples of a current 64×64 coding region, while another 64×64 referencesample memory block is used to store a previously coded 64×64 codingregion.

FIG. 9 describes an example of a coding order and reference samplememory usage.

The hatched region is the 64×64 region, which includes the currentcoding block. The shaded region(s) are already coded 64×64 regions inthe current or left CTU. The hatched 64×64 regions have not yet beencoded. When the current coding block falls into one of the four 64×64regions in the current CTU, the reference sample memory can storeanother 64×64 coded region for the reference of the IBC mode. Thecurrent 64×64 region, together with the other 64×64 reference region,are indicated by a dotted rectangular in FIG. 9.

The top row of FIG. 9 shows an exemplary coding order of each 64×64region under horizontal binary split or quad-tree split at a 128×128level. When the current block (911) is a top-left block of the currentCTU, the reconstructed block (912) can be determined to be a block ofanother CTU, such as a top-right block of another CTU, which is to theleft of the CTU. Therefore, if a current sub-block (current codingblock) falls into the top-left 64×64 block (911) of the current CTU,then in addition to the already reconstructed samples in the currenttop-left 64×64 region (911), the reconstruction of the current sub-blockcan also refer to the reference samples in the left 64×64 block (912),which is in the CTU to the left of the current CTU, using the IBC mode.

When the current block (921) is a top-right block of the current CTU,the reconstructed block (922) can be determined to be another block ofthe same CTU, such as a top-left block of the same CTU. Therefore, if acurrent sub-block falls into the top-right 64×64 block (921) of thecurrent CTU, then in addition to the already reconstructed samples inthe current top-right 64×64 region (921), the reconstruction of thecurrent sub-block can also refer to the reference samples in thetop-left 64×64 block (922) of the current CTU, using the IBC mode.

When the current block (931) is a bottom-left block of the current CTU,the reconstructed block (932) can be determined to be another block ofthe same CTU, such as a top-right block of the same CTU. Therefore, if acurrent sub-block falls into the bottom-left 64×64 block (931) of thecurrent CTU, then in addition to the already reconstructed samples inthe current bottom-left 64×64 region (931) of the current CTU, thereconstruction of the current sub-block can also refer to the referencesamples in the top-right 64×64 block (932) of the current CTU, using theIBC mode.

When the current block (941) is a bottom-right block of the current CTU,the reconstructed block (942) can be determined to be another block ofthe same CTU, such as a top-right block of the same CTU. Therefore, if acurrent sub-block falls into the bottom-right 64×64 block (941) of thecurrent CTU, then in addition to the already reconstructed samples inthe current bottom-right 64×64 region (941) of the current CTU, thereconstruction of the current sub-block can also refer to the referencesamples in the top-right 64×64 block (942) of the current CTU, using theIBC mode.

FIG. 9 just provides exemplary reference area assignments, otherpossible reference area assignments, such as using the top-right 64×64region as the reference area for the bottom-right 64×64 region (labeled“3”), are also within the scope of this disclosure.

The bottom row of FIG. 9 shows an exemplary coding order of each 64×64region under vertical binary split or quad-tree split at 128×128 level.When the current block (951) is a top-left block of the current CTU, thereconstructed block (952) can be determined to be a block of anotherCTU, such as a top-right block of another CTU, which is to the left ofthe CTU. Therefore, if a current sub-block falls into the top-left 64×64block (951) of the current CTU, then in addition to the alreadyreconstructed samples in the current top-left 64×64 region (951), thereconstruction of the current sub-block can also refer to the referencesamples in the left 64×64 block (952), which is in the CTU to the leftof the current CTU, using the IBC mode.

When the current block (961) is a bottom-left block of the current CTU,the reconstructed block (962) can be determined to be another block ofthe same CTU, such as a top-left block of the same CTU. Therefore, if acurrent sub-block falls into the bottom-left 64×64 block (961) of thecurrent CTU, then in addition to the already reconstructed samples inthe current bottom-left 64×64 region (961) of the current CTU, thereconstruction of the current sub-block can also refer to the referencesamples in the top-left 64×64 block (962) of the current CTU, using theIBC mode.

When the current block (971) is a top-right block of the current CTU,the reconstructed block (972) can be determined to be another block ofthe same CTU, such as a top-left block of the same CTU. Therefore, if acurrent sub-block falls into the top-right 64×64 block (971) of thecurrent CTU, then in addition to the already reconstructed samples inthe current top-right 64×64 region (971), the reconstruction of thecurrent sub-block can also refer to the reference samples in thetop-left 64×64 block (972) of the current CTU, using the IBC mode.

When the current block (981) is a bottom-right block of the current CTU,the reconstructed block (982) can be determined to be another block ofthe same CTU, such as a top-right block of the same CTU. Therefore, if acurrent sub-block falls into the bottom-right 64×64 block (981) of thecurrent CTU, then in addition to the already reconstructed samples inthe current bottom-right 64×64 region (981) of the current CTU, thereconstruction of the current sub-block can also refer to the referencesamples in the top-right 64×64 block (982) of the current CTU, using theIBC mode.

Other assignments can be made in a similar fashion. For example, the64×64 reference area may be another already coded area.

Table 1 below summarizes availabilities of reference samples from the64×64 region other than the current 64×64 region in view of FIG. 9. TL,TR, BL and BR refer to top-left, top-right, bottom-left andbottom-right, respectively. The mark “X” means not available, and themark “Y” means available.

TABLE 1 Reference sample availability from left CTU Ref samples CurrentCurrent Current Current Current Left CTU CTU CTU CTU CTU samples in TRTL TR BL BR current CTU 64 × 64 64 × 64 64 × 64 64 × 64 64 × 64 TL 64 ×64 Y X X X X TR 64 × 64 N Y X X X BL 64 × 64 X Y (if Y (if X X verticalBT horizontal at 128 × 128 BT or QT level) at 128 × 128 level) BR 64 ×64 X X Y X X

In the above example, whether a reference block for the current block inIBC mode is in the left CTU can be determined based on (i) whether allthe samples in the reference block are from the left CTU or (ii) whetherone or more reference samples in the reference block is from the leftCTU.

II. Reference Sample Memory with Three Sets of 64×64 Luma Samples

In some embodiments of the present disclosure, the maximum size of amemory that can be used to store IBC reference samples is three sets of64×64 luma samples and corresponding chroma samples. The followingdiscussion provides method for utilizing this memory size to perform theIBC mode.

In one example, one 64×64 reference sample memory block is used to storesamples of a current 64×64 coding region, while another two 64×64 sizedreference sample memory blocks is used to store previously coded two64×64 coding regions as additional reference areas for the IBC.

As shown in FIG. 10, the hatched region in each of the CTU is the 64×64region, which includes the current coding block. The shaded regions arealready coded 64×64 regions in the current or left CTU, while thehatched 64×64 region has not yet been coded. When the current codingblock falls into one of the four shaded 64×64 regions in the currentCTU, the reference sample memory can store another two 64×64 codedregions for the reference of IBC. The current 64×64 region, togetherwith the other two 64×64 reference regions, are indicated by a dottedrectangular in FIG. 10.

The top row of FIG. 10 shows a coding order of each 64×64 region under ahorizontal binary split or quad-tree split, for example at a 128×128level. When the current block (1011) is a top-left block of the currentCTU, the reconstructed blocks (1012 a, 1012 b) can be blocks of anotherCTU, such as a top-right block of another CTU and a bottom-right blockof the other CTU, which is to the left of the CTU. Therefore, if acurrent sub-block falls into the top-left 64×64 block (1011) of thecurrent CTU, then in addition to the already reconstructed samples inthe current top-left 64×64 region (1011), the reconstruction of thecurrent sub-block can also refer to the reference samples in thetop-right 64×64 block and the bottom-right 64×64 block of the left CTU,using the IBC mode.

When the current block (1021) is a top-right block of the current CTU,the reconstructed blocks (e.g., 1022 a, 1022 b) can be blocks of anotherCTU and the same CTU, such as the top-left block of the current CTU anda top-right block of the other CTU, which is to the left of the CTU.Therefore, if a current sub-block falls into the top-right 64×64 block(1021) of the current CTU, then in addition to the already reconstructedsamples in the current top-right 64×64 region (1021), the reconstructionof the current sub-block can also refer to the reference samples in thetop-left 64×64 block of the current CTU and the top-right 64×64 block ofthe left CTU, using the IBC mode.

When the current block (1031) is a bottom-left block of the current CTU,the reconstructed blocks (e.g., 1032 a, 1032 b) can be blocks of thesame CTU, such as the top-left block and the top-right block of thecurrent CTU. Therefore, if a current sub-block falls into thebottom-left 64×64 block (1031) of the current CTU, then in addition tothe already reconstructed samples in the current bottom-left 64×64region (1031), the reconstruction of the current sub-block can alsorefer to the reference samples in the top-left 64×64 block and thetop-right 64×64 block of the current CTU, using the IBC mode.

When the current block is a bottom-right block (1041) of the currentCTU, the reconstructed blocks (e.g., 1042 a, 1042 b) can be blocks ofthe same CTU, such as a top-right block and the bottom-left block of thesame CTU. Therefore, if a current sub-block falls into the bottom-right64×64 block (1041) of the current CTU, then in addition to the alreadyreconstructed samples in the current bottom-right 64×64 region (1041) ofthe current CTU, the reconstruction of the current sub-block can alsorefer to the reference samples in the top-right 64×64 block and thebottom-left 64×64 block of the current CTU, using the IBC mode.

While FIG. 10 provides exemplary assignments, other possible referencearea assignments, such as using the top-right 64×64 region in the leftCTU as the reference area for the bottom-left 64×64 region of currentCTU (labeled “1”, in the bottom row), are also within the scope of thisdisclosure.

The bottom row of FIG. 10 shows an exemplary coding order of each 64×64region under a vertical binary split or quad-tree split, for example ata 128×128 level. When the current block (1051) is a top-left block ofthe current CTU, the reconstructed blocks (e.g., 1052 a, 1052 b) can beblocks of another CTU, such as a top-right block and a bottom-rightblock of the other CTU, which is to the left of the CTU. Therefore, if acurrent sub-block falls into the top-left 64×64 block (1051) of thecurrent CTU, then in addition to the already reconstructed samples inthe current top-left 64×64 region (1051), the reconstruction of thecurrent sub-block can also refer to the reference samples in thetop-right 64×64 block and the bottom-right 64×64 block of the left CTU,using the IBC mode.

When the current block is a bottom-left block (1061) of the current CTU,the reconstructed blocks (e.g., 1062 a, 1062 b) can be blocks of thesame CTU and another CTU, such as a top-left block of the same CTU and abottom-right block of the other CTU, which is to the left of the currentCTU. Therefore, if a current sub-block falls into the bottom-left 64×64block (1061) of the current CTU, then in addition to the alreadyreconstructed samples in the current bottom-left 64×64 region (1061) ofthe current CTU, the reconstruction of the current sub-block can alsorefer to the reference samples in the top-left 64×64 block of thecurrent CTU and the bottom-right 64×64 block of the left CTU, using theIBC mode.

When the current block (1071) is a top-right block of the current CTU,the reconstructed blocks (e.g., 1072 a, 1072 b) can be blocks of thesame CTU, such as a top-left block and a bottom-left block of the sameCTU. Therefore, if a current sub-block falls into the top-right 64×64block (1071) of the current CTU, then in addition to the alreadyreconstructed samples in the current top-right 64×64 region (1071), thereconstruction of the current sub-block can also refer to the referencesamples in the top-left 64×64 block and the bottom-left 64×64 block ofthe current CTU, using the IBC mode.

When the current block is a bottom-right block (1081) of the currentCTU, the reconstructed blocks (e.g., 1082 a, 1082 b) can be blocks ofthe same CTU, such as a top-right block and a bottom-left block of thesame CTU. Therefore, if a current sub-block falls into the bottom-right64×64 block (1081) of the current CTU, then in addition to the alreadyreconstructed samples in the current bottom-right 64×64 region (1081) ofthe current CTU, the reconstruction of the current sub-block can alsorefer to the reference samples in the top-right 64×64 block and thebottom-left 64×64 block of the current CTU, using the IBC mode.

In the above example, whether a reference block for the current block inIBC mode is in the left CTU can be determined based on (i) whether allthe samples in the reference block are from the left CTU or (ii) whetherone or more reference samples in the reference block is from the leftCTU.

Other assignments can be made in a similar fashion. For example, theother 64×64 reference area may be another already coded area.

III. Reference Sample Memory with One Set of 64×64 Luma Samples

As shown in FIG. 11, when the maximum size of a memory that can be usedto store intra block copy reference samples is one set of 64×64 lumasamples, this one 64×64 reference sample memory block can be used tostore samples of a current 64×64 coding region (1110), a reference block(1120) for the IBC mode is also from the same 64×64 region as thecurrent block. In one example, when the top-left corner and bottom-rightcorner of the reference block have been reconstructed, the wholereference block has been reconstructed, and the reconstruction of acurrent sub-block (current coding block) can refer to the referencesamples in the reference block. Both of the reference block and thecurrent sub-block are in the same current 64×64 region.

D. Decoding Process Using The Reference Sample Memory

FIG. 12 shows a flow chart outlining a decoding process (1200) accordingto an embodiment of the disclosure. The process (1200) can be used todecode a block (i.e., a current block) of a picture using IBC mode. Insome embodiments, one or more operations are performed before or afterprocess (1200), and some of the operations illustrated in FIG. 12 may bereordered or omitted. In various embodiments, the process (1200) isexecuted by processing circuitry, such as the processing circuitry inthe terminal devices (210), (220), (230), and (240), the processingcircuitry that performs functions of the video decoder (310), (410), or(710), and the like. In some embodiments, the process (1200) isimplemented by software instructions, thus when the processing circuitryexecutes the software instructions, the processing circuitry performsthe process (1200). The process starts at (S1201) and proceeds to(S1210).

At (S1210), reconstructed samples of a reconstructed block of a pictureare stored in a first reference sample memory. The first referencesample memory is configured to store at least one set of a number ofluma samples and corresponding chroma samples of the reconstructedblock. The number of luma samples may be 64×64. In some examples, thereconstructed blocks correspond to the reconstructed blocks described inFIGS. 9-10. In some examples, the reconstructed samples of thereconstructed block can be generated using the system or decodersillustrated in FIGS. 3, 4, and 7.

At (S1220), reconstructed samples of a current block of the picture arestored in a second reference sample memory. The second reference samplememory is configured to store only one set of the number of luma samplesand corresponding chroma samples of the current block. Additional setsof the number of luma samples and corresponding chroma samples of thecurrent block may be stored for a larger second reference sample memory.In some examples, the second reference sample memory only storesreconstructed samples of a 64×64 region. A reference sample memory mayinclude both, for example partitioned into, the first reference samplememory and the second reference sample memory.

At (S1230), a current sub-block in the current block is reconstructedusing an intra block copy (IBC) mode. The IBC mode is based on, forexample, the stored reconstructed samples of a reference sub-block ofthe reconstructed block or the stored reconstructed samples of areference sub-block of the current block in the first reference samplememory.

After (S1230), the process proceeds to (S1299) and terminates.

FIG. 13 shows a flow chart outlining a decoding process (1300) accordingto an embodiment of the disclosure. The process (1300) can be used todecode a block (i.e., a current block) of a picture using IBC mode. Insome embodiments, one or more operations are performed before or afterprocess (1300), and some of the operations illustrated in FIG. 13 may bereordered or omitted.

In various embodiments, the process (1300) is executed by processingcircuitry, such as the processing circuitry in the terminal devices(210), (220), (230), and (240), the processing circuitry that performsfunctions of the video encoder (303), (503), or (603), and the like. Insome embodiments, the process (1300) is implemented by softwareinstructions, thus when the processing circuitry executes the softwareinstructions, the processing circuitry performs the process (1300). Theprocess starts at (1301) and proceeds to (S1310).

At (S1310), reconstructed samples of a current block of a picture arestored in a reference sample memory. A size of the current block doesnot exceed one set of a number of luma samples and corresponding chromasamples according to one embodiment. In some examples, the number ofluma samples may be 64×64.

At (S1320), a current sub-block in the current block is reconstructedusing an intra block copy (IBC) mode. The IBC mode is based on, forexample, the stored reconstructed samples of a reference sub-block ofthe current block. In this case, a maximum size of the reference samplememory can be limited to the one set of a number of luma samples andcorresponding chroma samples. The number of luma samples may be 64×64and the one 64×64 reference sample memory block can be used to storesamples of a current 64×64 coding region. A reference block for the IBCmode may be also in the same 64×64 region. In some examples, the currentregion corresponds to the block (1310) and the reference blockcorresponds to the block (1320) in FIG. 11. In some examples, thereconstructed samples of the current block can be generated using thesystem or encoders illustrated in FIGS. 3, 5, and 6.

After (S1320), the process proceeds to (S1399) and terminates.

III. Computer System

The techniques described above, can be implemented as computer softwareusing computer-readable instructions and physically stored in one ormore computer-readable media. For example, FIG. 14 shows a computersystem (1400) suitable for implementing certain embodiments of thedisclosed subject matter.

The computer software can be coded using any suitable machine code orcomputer language, that may be subject to assembly, compilation,linking, or like mechanisms to create code comprising instructions thatcan be executed directly, or through interpretation, micro-codeexecution, and the like, by one or more computer central processingunits (CPUs), Graphics Processing Units (GPUs), and the like.

The instructions can be executed on various types of computers orcomponents thereof, including, for example, personal computers, tabletcomputers, servers, smartphones, gaming devices, internet of thingsdevices, and the like.

The components shown in FIG. 14 for computer system (1400) are exemplaryin nature and are not intended to suggest any limitation as to the scopeof use or functionality of the computer software implementingembodiments of the present disclosure. Neither should the configurationof components be interpreted as having any dependency or requirementrelating to any one or combination of components illustrated in theexemplary embodiment of a computer system (1400).

Computer system (1400) may include certain human interface inputdevices. Such a human interface input device may be responsive to inputby one or more human users through, for example, tactile input (such as:keystrokes, swipes, data glove movements), audio input (such as: voice,clapping), visual input (such as: gestures), olfactory input (notdepicted). The human interface devices can also be used to capturecertain media not necessarily directly related to conscious input by ahuman, such as audio (such as: speech, music, ambient sound), images(such as: scanned images, photographic images obtain from a still imagecamera), video (such as two-dimensional video, three-dimensional videoincluding stereoscopic video).

Input human interface devices may include one or more of (only one ofeach depicted): keyboard (1401), mouse (1402), trackpad (1403), touchscreen (1410), data-glove (not shown), joystick (1405), microphone(1406), scanner (1407), camera (1408).

Computer system (1400) may also include certain human interface outputdevices. Such human interface output devices may be stimulating thesenses of one or more human users through, for example, tactile output,sound, light, and smell/taste. Such human interface output devices mayinclude tactile output devices (for example tactile feedback by thetouch-screen (1410), data-glove (not shown), or joystick (1405), butthere can also be tactile feedback devices that do not serve as inputdevices), audio output devices (such as: speakers (1409), headphones(not depicted)), visual output devices (such as screens (1410) toinclude CRT screens, LCD screens, plasma screens, OLED screens, eachwith or without touch-screen input capability, each with or withouttactile feedback capability—some of which may be capable to output twodimensional visual output or more than three dimensional output throughmeans such as stereographic output; virtual-reality glasses (notdepicted), holographic displays and smoke tanks (not depicted)), andprinters (not depicted).

Computer system (1400) can also include human accessible storage devicesand their associated media such as optical media including CD/DVD ROM/RW(1420) with CD/DVD or the like media (1421), thumb-drive (1422),removable hard drive or solid state drive (1423), legacy magnetic mediasuch as tape and floppy disc (not depicted), specialized ROM/ASIC/PLDbased devices such as security dongles (not depicted), and the like.

Those skilled in the art should also understand that term “computerreadable media” as used in connection with the presently disclosedsubject matter does not encompass transmission media, carrier waves, orother transitory signals.

Computer system (1400) can also include an interface to one or morecommunication networks. Networks can for example be wireless, wireline,optical. Networks can further be local, wide-area, metropolitan,vehicular and industrial, real-time, delay-tolerant, and so on. Examplesof networks include local area networks such as Ethernet, wireless LANs,cellular networks to include GSM, 3G, 4G, 5G, LTE and the like, TVwireline or wireless wide area digital networks to include cable TV,satellite TV, and terrestrial broadcast TV, vehicular and industrial toinclude CANBus, and so forth. Certain networks commonly require externalnetwork interface adapters that attached to certain general purpose dataports or peripheral buses (1449) (such as, for example USB ports of thecomputer system (1400)); others are commonly integrated into the core ofthe computer system (1400) by attachment to a system bus as describedbelow (for example Ethernet interface into a PC computer system orcellular network interface into a smartphone computer system). Using anyof these networks, computer system (1400) can communicate with otherentities. Such communication can be uni-directional, receive only (forexample, broadcast TV), uni-directional send-only (for example CANbus tocertain CANbus devices), or bi-directional, for example to othercomputer systems using local or wide area digital networks. Certainprotocols and protocol stacks can be used on each of those networks andnetwork interfaces as described above.

Aforementioned human interface devices, human-accessible storagedevices, and network interfaces can be attached to a core (1440) of thecomputer system (1400).

The core (1440) can include one or more Central Processing Units (CPU)(1441), Graphics Processing Units (GPU) (1442), specialized programmableprocessing units in the form of Field Programmable Gate Areas (FPGA)(1443), hardware accelerators for certain tasks (1444), and so forth.These devices, along with Read-only memory (ROM) (1445), Random-accessmemory (1446), internal mass storage such as internal non-useraccessible hard drives, SSDs, and the like (1447), may be connectedthrough a system bus (1448). In some computer systems, the system bus(1448) can be accessible in the form of one or more physical plugs toenable extensions by additional CPUs, GPU, and the like. The peripheraldevices can be attached either directly to the core's system bus (1448),or through a peripheral bus (1449). Architectures for a peripheral businclude PCI, USB, and the like.

CPUs (1441), GPUs (1442), FPGAs (1443), and accelerators (1444) canexecute certain instructions that, in combination, can make up theaforementioned computer code. That computer code can be stored in ROM(1445) or RAM (1446). Transitional data can be also be stored in RAM(1446), whereas permanent data can be stored for example, in theinternal mass storage (1447). Fast storage and retrieve to any of thememory devices can be enabled through the use of cache memory, that canbe closely associated with one or more CPU (1441), GPU (1442), massstorage (1447), ROM (1445), RAM (1446), and the like.

The computer readable media can have computer code thereon forperforming various computer-implemented operations. The media andcomputer code can be those specially designed and constructed for thepurposes of the present disclosure, or they can be of the kind wellknown and available to those having skill in the computer software arts.

As an example and not by way of limitation, the computer system havingarchitecture (1400), and specifically the core (1440) can providefunctionality as a result of processor(s) (including CPUs, GPUs, FPGA,accelerators, and the like) executing software embodied in one or moretangible, computer-readable media. Such computer-readable media can bemedia associated with user-accessible mass storage as introduced above,as well as certain storage of the core (1440) that are of non-transitorynature, such as core-internal mass storage (1447) or ROM (1445). Thesoftware implementing various embodiments of the present disclosure canbe stored in such devices and executed by core (1440). Acomputer-readable medium can include one or more memory devices orchips, according to particular needs. The software can cause the core(1440) and specifically the processors therein (including CPU, GPU,FPGA, and the like) to execute particular processes or particular partsof particular processes described herein, including defining datastructures stored in RAM (1446) and modifying such data structuresaccording to the processes defined by the software. In addition or as analternative, the computer system can provide functionality as a resultof logic hardwired or otherwise embodied in a circuit (for example:accelerator (1444)), which can operate in place of or together withsoftware to execute particular processes or particular parts ofparticular processes described herein. Reference to software canencompass logic, and vice versa, where appropriate. Reference to acomputer-readable media can encompass a circuit (such as an integratedcircuit (IC)) storing software for execution, a circuit embodying logicfor execution, or both, where appropriate. The present disclosureencompasses any suitable combination of hardware and software.

APPENDIX A: ACRONYMS

JEM: joint exploration modelVVC: versatile video codingBMS: benchmark set

MV: Motion Vector HEVC: High Efficiency Video Coding SEI: SupplementaryEnhancement Information VUI: Video Usability Information GOPs: Groups ofPictures TUs: Transform Units, PUs: Prediction Units CTUs: Coding TreeUnits CTBs: Coding Tree Blocks PBs: Prediction Blocks HRD: HypotheticalReference Decoder SNR: Signal Noise Ratio CPUs: Central Processing UnitsGPUs: Graphics Processing Units CRT: Cathode Ray Tube LCD:Liquid-Crystal Display OLED: Organic Light-Emitting Diode CD: CompactDisc DVD: Digital Video Disc ROM: Read-Only Memory RAM: Random AccessMemory ASIC: Application-Specific Integrated Circuit PLD: ProgrammableLogic Device LAN: Local Area Network

GSM: Global System for Mobile communications

LTE: Long-Term Evolution CANBus: Controller Area Network Bus USB:Universal Serial Bus PCI: Peripheral Component Interconnect FPGA: FieldProgrammable Gate Areas

SSD: solid-state drive

IC: Integrated Circuit CU: Coding Unit IBC: Intra Block Copy CPR:Current Picture Referencing BV: Block Vector AMVP: Advanced MotionVector Prediction HEVC SCC: HEVC Screen Content Coding DPB: DecodedPicture Buffer QT: Quaternary-Tree BT: Binary-Tree TT: Ternary-Tree

TL: top-leftTR: top-rightBL: bottom-leftBR: bottom-right

While this disclosure has described several exemplary embodiments, thereare alterations, permutations, and various substitute equivalents, whichfall within the scope of the disclosure. It will thus be appreciatedthat those skilled in the art will be able to devise numerous systemsand methods which, although not explicitly shown or described herein,embody the principles of the disclosure and are thus within the spiritand scope thereof.

What is claimed is:
 1. A method for video decoding in a decoder,comprising: storing reconstructed samples of a reconstructed block of apicture in a first reference sample memory, the first reference samplememory being configured to store at least one set of a number of lumasamples and corresponding chroma samples of the reconstructed block;storing reconstructed samples of a current block of the picture in asecond reference sample memory, the second reference sample memory beingconfigured to store only one set of the number of luma samples andcorresponding chroma samples of the current block; and reconstructing acurrent sub-block in the current block using an intra block copy (IBC)mode based on the stored reconstructed samples of a reference sub-blockof the reconstructed block or the stored reconstructed samples of areference sub-block of the current block.
 2. The method of claim 1,wherein each of the at least one set of the luma samples of thereconstructed block and the one set of the luma samples of the currentblock includes 64×64 luma samples.
 3. The method of claim 1, wherein amaximum size of the first reference sample memory is limited to a sizeof two sets of the luma samples and the corresponding chroma samples. 4.The method of claim 1, wherein a coding tree unit (CTU) is partitionedinto one or more non-overlapping blocks, the one or more non-overlappingblocks including the current block, and the reconstructed block isdetermined based on a location of the current block relative to the CTUand a decoding order of the one or more non-overlapping blocks.
 5. Themethod of claim 4, wherein when the current block is a top-left block ofthe CTU, the reconstructed block is determined to be a top-right blockof another CTU, the other CTU being to the left of the CTU.
 6. Themethod of claim 4, wherein when the current block is a top-right blockof the CTU or a bottom-left block of the CTU, the reconstructed block isdetermined to be a top-left block of the CTU.
 7. The method of claim 4,wherein when the current block is a bottom-right block of the CTU, thereconstructed block is determined to be a top-right block of the CTU. 8.The method of claim 4, wherein when the current block is a bottom-leftblock of the CTU, the reconstructed block is determined to be atop-right block of the CTU.
 9. The method of claim 1, furthercomprising: storing reconstructed samples of another reconstructed blockof the picture in the first reference sample memory, a size of the otherreconstructed block not exceeding one set of the the number of lumasamples and corresponding chroma samples; and reconstructing the currentsub-block in the current block using the IBC mode based on the storedreconstructed samples of the reference sub-block of the reconstructedblock, the stored reconstructed samples of a reference sub-block of theother reconstructed block, or the stored reconstructed samples of thereference sub-block of the current block.
 10. The method of claim 4,wherein when the current block is a top-left block of the CTU, thereconstructed block is determined to be a top-right block and abottom-right block of another CTU, the other CTU being to the left ofthe CTU.
 11. A method for video decoding in a decoder, comprising:storing reconstructed samples of a current block of a picture in areference sample memory, a size of the current block not exceeding oneset of luma samples and corresponding chroma samples; and reconstructinga current sub-block in the current block using an intra block copy (IBC)mode based on the stored reconstructed samples of a reference sub-blockof the current block, wherein a maximum size of the reference samplememory is limited to the one set of luma samples and correspondingchroma samples.
 12. An apparatus, comprising: processing circuitryconfigured to: store reconstructed samples of a reconstructed block of apicture in a first reference sample memory, the first reference samplememory being configured to store at least one set of a number of lumasamples and corresponding chroma samples of the reconstructed block,store reconstructed samples of a current block of the picture in asecond reference sample memory, the second reference sample memory beingconfigured to store only one set of the number of luma samples andcorresponding chroma samples of the current block, and reconstruct acurrent sub-block in the current block using an intra block copy (IBC)mode based on the stored reconstructed samples of a reference sub-blockof the reconstructed block or the stored reconstructed samples of areference sub-block of the current block.
 13. The apparatus of claim 11,wherein each of the at least one set of the luma samples of thereconstructed block and the one set of the luma samples of the currentblock includes 64×64 luma samples.
 14. The apparatus of claim 11,wherein a maximum size of the first reference sample memory is limitedto a size of two sets of the luma samples and the corresponding chromasamples.
 15. The apparatus of claim 11, wherein a coding tree unit (CTU)is partitioned into one or more non-overlapping blocks, the one or morenon-overlapping blocks including the current block, and thereconstructed block is determined based on a location of the currentblock relative to the CTU and a decoding order of the one or morenon-overlapping blocks.
 16. The apparatus of claim 15, wherein when thecurrent block is a top-left block of the CTU, the reconstructed block isdetermined to be a top-right block of another CTU, the other CTU beingto the left of the CTU.
 17. The apparatus of claim 11, wherein theprocessing circuitry is further configured to store reconstructedsamples of another reconstructed block of the picture in the firstreference sample memory, a size of the other reconstructed block notexceeding one set of the the number of luma samples and correspondingchroma samples, and reconstruct the current sub-block in the currentblock using the IBC mode based on the stored reconstructed samples ofthe reference sub-block of the reconstructed block, the storedreconstructed samples of a reference sub-block of the otherreconstructed block, or the stored reconstructed samples of thereference sub-block of the current block.
 18. An apparatus, comprising:processing circuitry configured to: store reconstructed samples of acurrent block of a picture in a reference sample memory, a size of thecurrent block not exceeding one set of luma samples and correspondingchroma samples; and reconstruct a current sub-block in the current blockusing an intra block copy (IBC) mode based on the stored reconstructedsamples of a reference sub-block of the current block, wherein a maximumsize of the reference sample memory is limited to the one set of lumasamples and corresponding chroma samples.
 19. A non-transitorycomputer-readable medium storing instructions which when executed by acomputer for video decoding cause the computer to perform: storingreconstructed samples of a reconstructed block of a picture in a firstreference sample memory, the first reference sample memory beingconfigured to store at least one set of a number of luma samples andcorresponding chroma samples of the reconstructed block; storingreconstructed samples of a current block of the picture in a secondreference sample memory, the second reference sample memory beingconfigured to store only one set of the number of luma samples andcorresponding chroma samples of the current block; and reconstructing acurrent sub-block in the current block using an intra block copy (IBC)mode based on the stored reconstructed samples of a reference sub-blockof the reconstructed block or the stored reconstructed samples of areference sub-block of the current block.
 20. A non-transitorycomputer-readable medium storing instructions which when executed by acomputer for video decoding cause the computer to perform: storingreconstructed samples of a current block of a picture in a referencesample memory, a size of the current block not exceeding one set of lumasamples and corresponding chroma samples; and reconstructing a currentsub-block in the current block using an intra block copy (IBC) modebased on the stored reconstructed samples of a reference sub-block ofthe current block, wherein a maximum size of the reference sample memoryis limited to the one set of luma samples and corresponding chromasamples.